K as in Constant

When Texas Instruments introduced in September 1971 with the TMS1802 the first "calculator-on-a-chip", it did not even sport a [=] key. Its [+=] and [−=] keys clearly demonstrated that the Integrated Circuit (IC) was designed to replace adding machines used in offices and not slide rules. Consequently were these two keys used to accumulate numbers in a register and some calculator designs even labeled the Clear key with [CA] for Clear Accumulator. With Adding Machine Logic the [+=] and [−=] keys always complete operations, meaning the key sequence [2] [x] [3] [+=] [4] [x] [5] [+=] is resulting in 20 from the evaluation of 4 x 5, the previous calculation of 2 x 3 = 6 was cleared in the moment the [4] key was pressed.

The [K] or [CHAIN-CONST] switch often used with TMS0100 single-chip calculator circuit designs is using the Constant Mode to simplify repeating multiplications and divisions but does not affect the outcome of the key sequence illustrated above. K as in Constant? Not really. Texas Instruments introduced together with the TMS1802 all major components to design and build an electronic calculator, including the TIL360 LED display modules, matching SN75491 LED segment and SN75492 digit drivers and 1KS/6KS Klixon™ keyboards. The TMS1802 itself is using a keyboard scanning logic with 11 outputs and 4 inputs, named KN for the numeral keys, KO for the operation keys (functions), KP for the decimal position selector switch and KQ for "other switches", like Constant Mode. For whatever reasons decided Texas Instruments to mark this switch on the Klixon keyboard with "K" and since then we know the [K] switch to select the Constant Mode. And with the instant success of the Bowmar 901B pocket calculator based on the Texas Instruments "calculator kit", the word was spreading quickly. Obviously a confusing naming convention and Texas Instruments used with their first own calculator design, the famous TI-2500 Datamath, a less confusing [CHAIN-CONST] label for the switch, yet using the [K] moniker for products manufactured for third-parties, like the Radio Shack EC-200. The damage was already done and even calculators omitting the Klixon keyboard sometimes use a [K] switch like the Commodore Minuteman 2.

Later members of the TMS0100 single-chip calculator family like the TMS0119 used with the TI-2500 Datamath calculator, introduced the concept of a separate [=] key to allow mixed calculations with combinations of addition, subtraction, multiplication and division in Chain Mode. The calculator will be cleared pressing a number key after the [=] key. The key sequence [2] [x] [3] + [4] [x] [5] [=] will be evaluated in the order of its entry and consequently resulting in 50 from the evaluation 2 x 3 = 6, 6 + 4 = 10 and 10 x 5 = 50. T.

Later algorithm designs omitted the costly [CHAIN-CONST] switch and started using the concept of an Automatic Constant, meaning the answer from a multiplication or division is entered as a Constant with the [=] key. For practical reasons most implementation use for Multiplications the 1^st operand as a Constant and for Divisions, Additions, and Subtractions the 2^nd operand as a Constant. Here at the Datamath Calculator Museum we annotate this implementation as M-D-A-S (1-2-2-2). This approach mimics old-school Multiplication and Division Table Charts used in Elementary Schools all over the world. The Constant Mode feature simplifies raising a number to a power and calculating the reciprocal of a number, too.

Example operations for a calculator with M-D-A-S (1-2-2-2) Automatic Constant: